Threaded joints have been used as joints for such oil well pipes. The most commonly used type is a round thread joint or a buttress thread joint meeting API specifications (American Petroleum Institute specifications) (also referred to below as an “API joint”). Recently, oil wells, gas wells, and the like are becoming deeper, and at the same time, the pressure, the temperature, and the corrosive conditions inside wells are becoming increasingly severe. For such environments, there has come to be a demand for a joint having a strength which can resist breakage under the weight of pipes and which also has excellent airtightness.
As one example thereof, FIG. 1a is a cross-sectional view showing a coupling-type threaded joint 25 for an oil well pipe for connecting a coupling 20 having box portions 21, 21 at each end and pipes 10, 10 having pin portions 11, 11 at the ends thereof. FIG. 1b is an enlarged cross-sectional view of a box portion 21 of the coupling 20, and FIG. 1c is an enlarged cross-sectional view of the pin portion 11 of a pipe 10.
As shown in FIG. 1a, the box portions 21, 21 are provided at both ends of the coupling 20, and the pin portions 11, 11 are provided on the ends of the pipes 10, 10. The pipes 10, 10 which are steel pipes in the case of oil well pipes are threadingly connected by the coupling 20.
As shown in FIG. 1b, a female thread 22 is formed on the inner peripheral surface of the box portion 21. In addition, as shown in FIG. 1c, a male thread 12 is formed on the outer peripheral surface of the pin portion 11.
In this manner, a coupling-type threaded joint 25 connects two pipes 10, 10 by screwing the pin portions 11 having male threads 12 provided on the ends of the pipes 10 into box portions 21 having female threads 22 provided on the interior of the coupling 20.
However, as stated above, a joint comprising only threaded elements in the form of males threads 12 and female threads 22 such as an API joint was not sufficient to meet the demands of a threaded joint 25 having a sufficient strength to resist tensile stresses generated by the weight of the pipes 10 and having a high degree of airtightness. Therefore, as respectively shown in FIG. 1b and FIG. 1c, a threaded joint 25 having metal seal portions 13, 23 and torque shoulder portions 14, 24 on the box portion 21 and the pin portion 11, respectively, has come to be used.
The metal seal portions 13, 23 are provided in order to guarantee a suitable contact surface pressure. The pipes 10 have interference in the radial direction, i.e., the outer diameter of the metal seal portions 13 of the pin portions 11 is larger than the inner diameter of the metal seal portions 23 of the box portions 21 (this difference is called the “interference amount”). When the pin portions 11 are screwed into the box portions 21, due to the existence of this interference amount, a surface pressure is generated in the contact surfaces of both metal seal portions 13, 23, and good airtightness is guaranteed by this surface pressure.
The torque shoulders 14, 24 are provided in order to guarantee a suitable contact surface pressure between them. Namely, by abutting these portions of the pin portion 11 and the box portion 21 against each other with a suitably controlled pressure, a sufficient amount of threaded engagement is guaranteed, coupling of the threaded joint 25 is performed with certainty, and due to the abutting force, a supplemental contact surface pressure is generated in the metal seal portions 13 and 23.
In the case of a tapered thread, in order to perform coupling with certainty and to prevent ready loosening, there are many cases in which an interference in the same amount as that in the metal seal portions 13, 23 is provided between the male thread 12 and the female thread 22.
The angle of slope θ1 of the torque shoulders 14, 24 with respect to a perpendicular surface is referred to as the shoulder angle.